Class of Pure Piezoelectric Materials

ABSTRACT

A modification of PbTiO 3  perovskite wherein at least part of Pb is replaced by a smaller atom with a similar ionic charge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/064,550, filed Mar. 11, 2008, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with U.S. Government support under grant N0001-4-01-1-0365 from the Office of Naval Research. The U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a novel class of piezoelectric and ferroelectric materials.

BACKGROUND OF THE INVENTION

Lead-zirconate-titanate (PZT)-based ceramics have been the primary material used in piezoelectric devices over the past several decades, especially in piezoelectric devices such as ultrasound medical probes, hydrophones and sonar for underwater imaging and communications, multi-layer actuators for fuel injection, piezoelectric printers etc. To enhance the piezoelectric properties of PZT ceramics, compositions near a phase transition are chosen.

In this regard, single crystals of solid solutions of lead-zinc-niobate and lead-titanate (PZN-PT) and of lead magnesium-niobate and lead-titanate (PMN-PT) are promising new materials. Compared to the conventional state-of-the-art PZT ceramics, PZN-PT and PMN-PT single crystals have significantly large electromechanical coupling (conversion of energy from electrical to mechanical and vice-versa) due to the much higher values of piezoelectric coefficients (about 4-5 times that of PZT), to offer advantages over the older standards.

Notwithstanding the advantages offered by newer piezoelectric materials such as PZN-PT and PMN-PT, there remains a desire to have further new piezoelectric materials with unique and advantageous characteristics, e.g. materials with piezoelectric coefficients which are significantly higher than those obtained with currently available materials.

SUMMARY OF THE INVENTION

The invention contemplates a novel class of piezoelectrics and ferroelectrics based on the use of chemical pressure to shift the high-pressure extremely high coupling transition recently found in PbTiO₃, to lower pressures. A particularly preferred compound within this group is Pb_(1/2)Sn_(1/2)TiO₃ (referred to elsewhere herein for convenience as PSnT). This new compound is considered to have giant piezoelectric coefficients, significantly higher than any currently known materials.

The class of ferroelectric and piezoelectric materials constituting the present invention is based on the concept of applying “chemical pressure” to tune (shift) the morphotropic phase boundary of pure PbTiO₃ [1,2] to lower pressures. Application of “chemical pressure” was performed by substituting the ‘A’ site of PbTiO₃ perovskite by smaller atoms with similar ionic charge, e.g. Sn, Ge or the like. Examples of the compounds thus obtainable are Pb_(1/2)Sn_(1/2)TiO₃, Pb_(1/2)Ge_(1/2)TiO₃ and Sn_(1/2)Ge_(1/2)TiO₃. These compounds represent a class of materials having superior electromechanical properties compared to previously available piezoelectric materials (FIG. 1 and FIG. 2).

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the structure of Pb_(1/2)Sn_(1/2)TiO₃ (also referred to herein as PbSnT);

FIG. 2 illustrates the pattern of two other compounds according to the invention.

FIG. 3 illustrates the Electric Enthalpy difference (meV) versus E_(z) (mV/Angstrom) between the phases of PSnT. The intersection with the zero axis shows the required field to rotate the polarization between the phases, The Cm to P4mm transition rotates the polarization and has a large strain, giving a giant electromechanical coupling d₃₃˜2100 pC/N. The results show that this transition is highly non-linear.

DETAILED DESCRIPTION OF THE INVENTION

Ab-initio simulations were performed using the LDA exchange-correlation functional with the ABINIT [3] package. The pseudopotential method was used to treat the core and the valence electrons. Full substitution of Pb by the smaller Sn atom to get SnTiO₃ gives a tetragonal ground state, with the rhombohedral phase at ˜12.6 meV/at higher energy. Substitution of the ‘A’ site with only 50% of Sn to form layered compounds of Pb_(1/2)Sn_(1/2)TiO₃, resulted in a more promising material that has been previously reported in the recent Nature paper [2]. Local density approximation computations using ABINIT show that it has an orthorhombic ground state of symmetry Pmm2, (polarization along [x00], c/a=0.91) with an energy difference of 12 meV/at between Pmm2 and the next state Cm (polarization along [xxz], c/a=0.98) followed by tetragonal P4mm (polarization along [00z], c/a=1.12) with a ΔE of 3 meV/at. Strain differences ε₃₃−ε₁₁ are predictably equal to −0.10, −0.02 and 0.11 in the three phases respectively. The Polarization (P), as computed by the Berry's phase method, in the P4mm phase is (0, 0, 1.2) C/m² and in its ground State Pmm2 phase is (1.1, 0, 0) C/m². For comparison, PZT P4mm phase has P=(0, 0, 0.81) C/m². The large predicted polarization implies a large Curie temperature T_(c).

The orthorhombic and the monoclinic phases are found to be dynamically stable. One can define an electric enthalpy H=U−P.E, where ‘U’ is the total energy and ‘P’ and ‘E’ are the polarization and electric field. FIG. 3 shows the enthalpy versus electric field along ‘z’ direction for the Cm phase with respect to the ground state and the P4mm phase with respect to the Cm phase. The zero crossing of the curves show that the critical field is very high to go from the Pmm2 phase to the Cm phase, but to rotate the polarization from the Cm to the P4mm phase one needs an electric field of only about ˜500 kV/cm. Using the strain of P4mm and Cm phase one obtains a d₃₃˜2100 pC/N. This is three times that of PZT-5H (largest component d₁₅˜741 pC/N) and comparable to that of PZN-PT (largest component d₃₃˜2000-3000 pC/N). It also shows a huge dielectric response (Table 2), with values close to that of PZT near its MPS composition. The large values of the polarization, dielectric constant and piezoelectric constants suggest that it may also have large non-linear optic coefficients.

The ground state of PSnT ordered along (001) is considered to be orthorhombic, and does not have giant piezoelectric coefficients. Rather the monoclinic Cm phase is believed to have giant coupling. The Cm phase is stabilized by growing PSnT epitaxially on SrTiO₃. In the monoclinic phase Pb_(1/2)Sn_(1/2)TiO₃ has a lattice parameter a˜7.44 a.u., very close to the lattice parameter of SrTiO₃ (a˜7.40 a.u ), so that it can be grown epitaxially using conventional MBE methods over SrTiO₃ and make it the ground state.

Disorder in Pb and Sn may also stabilize a monoclinic or rhombohedral ground state. Accordingly, it is considered that conventional crystal growth would also yield extraordinary electromechanical materials.

In addition to Pb_(1/2)Sn_(1/2)TiO₃, Pb_(1/2)Ge_(1/2)TiO₃ and Sn_(1/2)Ge_(1/2)TiO₃ have been found to be promising new materials. Pb_(1/2)Ge_(1/2)TiO₃ and Sn_(1/2)Ge_(1/2)TiO₃ both have a rhombohedral ground state in the rock salt pattern (space group R3m) with the tetragonal phase (space group I4mm) higher up in energy by 10.1 meV/at and 17.7 meV/at respectively. Their polarizations in the P4mm phase are (0, 0, 1.3) C/m² and (0, 0, 1.5) C/m² respectively.

The exact values of the lattice constants, strain, polarization and piezoelectric coefficients are sensitive to the choice of exchange-correlation functional, plane wave cutoff and k-point grids used to integrate over the Brillouin zone. However, these changes will not negatively affect the superior electromechanical properties of the present materials, these properties being greater than those of presently used transducer materials. It is believed that the simplicity of the new compounds of the invention will lower costs, which in turn should expand the applicability of piezoelectric materials. The large values of the polarization, dielectric constant and piezoelectric constants seem to suggest its use in making non-linear optic devices as well.

The design of a pure compound with giant electromechanical coupling will have significant advantages over relaxor ferroelectric solid solutions which require close control of composition and are heterogeneous. It should be much easier, for example, to grow ordered homohogeneous films of PSnT than PZN-PT or PMN-PT. If instead even higher coupling is desired, that may be achieved by alloying PSnT with another component and engineering the composition, as is done with PZT.

It will be appreciated that there are many uses for the materials of the invention. For example, one application of thin films of PSnT grown on SrTiO₃ is the wings of artificial insects, which can be used for surveillance, monitoring of hazardous environments such as in reactors or other high radiation areas, or in toxic locations. Another use of the present materials is as micropumps in medical applications. Numerous other applications in MEMS exist. Bulk samples can be used as high performance piezoelectrics wherever PZT or relaxor ferroelectrics are currently used.

TABLE 1 The coordinates in the monoclinic phase for Pb_(1/2)Sn_(1/2)TiO₃ are as below: R₁ (a.u) 7.447 0.066 0.014 R₂ (a.u) 0.066 7.447 0.014 R₃ (a.u) 0.028 0.028 14.538 Reduced coordinates for: Pb 0.031 0.031 0.014 Sn 0.080 0.080 0.516 Ti1 0.526 0.526 0.255 Ti2 0.523 0.523 0.755 O1 0.477 −0.013 0.248 O2 0.465 −0.015 0.742 O3 0.480 0.480 −0.001 O4 0.465 0.465 0.500 O5 −0.013 0.477 0.248 O6 −0.015 0.465 0.742

TABLE 2 Dielectric constants for PSnT in the Cm phase: Direction Dielectric constant X 651 Y 651 Z 238

The references cited above are more fully identified as follows:

1. Z. Wu and R. E. Cohen, Phys. Rev. Lett., 95, 37601 (2005)

2. Muhetaer Ahart and Maddury Somayazulu and R. E. Cohen and P. Ganesh and Przmeyslaw Dera and Ho-Kwang Mao and Russell J. Hemley and Yang Ren and Peter Liermann and Zhigang Wu, Nature 451, 06459 (2008)

3. X. Gonze et. al. Comput. Mater. Sci. 25, 478 (2002)

The contents of each of the above-listed references are incorporated herein in their entirety.

It will be appreciated that various modifications may be made in the invention as described above. Accordingly, the scope of the invention is set out in the following claims wherein: 

1. A modification of PbTiO₃ perovskite wherein at least part of Pb is replaced by a smaller atom with a similar ionic charge.
 2. The modification of claim 1 wherein the smaller atom is Sn and/or Ge.
 3. A compound selected from the group consisting of Pb_(1/2)Sn_(1/2)TiO₃, Pb_(1/2)Ge_(1/2)TiO₃ and Sn_(1/2)Ge_(1/2)TiO₃.
 4. A thin film of the compound of claim 3, wherein the thin film is grown on a substrate.
 5. The thin film of claim 4, wherein the substrate is SrTiO₃.
 6. A thin film of the modification of claim 1, wherein the thin film is grown on a substrate.
 7. The thin film of claim 6, wherein the substrate is SrTiO₃.
 8. Disorder, doping, or alloying of the materials of claims 1, 2 or 3 to enhance the electromechanical response. 